Modern rackets, used for tennis or for that matter, other sports including hand ball, squash and badminton, can be made from a wide variety of materials including laminated wood, steel tubing, aluminum, fiberglass, boron, graphite, as well as various combinations of these materials.
While experience over the years has shown that wood rackets absorb shock vibrations, it is relatively less strong than many of the other racket materials listed above. Thus, to produce a very rigid hitting platform or racket head in order to produce the greatest amount of power, very stiff rackets built from graphite or steel would be preferable.
On the other hand, if a person has tennis elbow or wishes to minimize elbow damaging vibrations occasioned by the racket head's contact with the ball, especially when very hard ground strokes are contemplated, then a more flexible racket structure would be preferred.
Over the years, the art has addressed various problems in attempting to vary the hitting force of racket strokes or in effect, to control the stiffness of a racket. Exemplary of this type of effort are Morrison, U.S. Pat. No. 1,587,918; Staub, British Pat. No. 498,430; Theodores et al, U.S. Pat. No. 4,105,205 and Dobo et al, German Pat. No. 2,843,640.
Morrison varies the hitting force during the stroke of his racket by incorporating a movable weight within a hollow handle that could be moved either towards or away from the elliptical head portion which he hoped could meet the particular stroke requirements for a particular user. The weight can be moved from a substantially neutral position immediately beneath the hand grip of the user to a position adjacent the head of the racket where the weight would provide an enlarged hitting force. The weight itself is prevented from lateral movement as is the threaded rod on which the rod is mounted.
In Staub, a hollow body is incorporated in the hollow racket handle so that it can be moved axially along the handle. When moved, it will displace a paste formed from inert, non-volatile liquids, such as glycerine which has been mixed together with a powder such as a pulverized metal. A threaded rod connects the hollow body to a disk provided at the base of the handle and by turning that disk the threaded shaft is rotated which positions the hollow body within the handle and thus varies the displacement of the paste. The percussion effect of the racket is changed as the paste material is moved so that as more paste is moved toward the head, the handle becomes relatively lighter and the percussion effect will be greater. When those conditions are reversed, with more paste lower in the handle the percussion effect will be less. In addition, an alternative embodiment suggests the inclusion of weights either at the base of the handle or adjacent the head of the racket which would again produce the same alternatives of having more weight lower in the handle or more toward the head, thereby modifying the percussion effect of the racket.
In Theodores et al, the stiffness of the racket is adjustable by using a shaped beam which has more flexibility in one direction than another. By rotating that beam, relative to the plane of the head of the racket, a more or less stiff effect can be achieved. One or more such beams can be used in the racket with the plurality being controlled by an interconnecting gear mechanism. The possibility is also suggested of incorporating a viscous damping fluid, such as a silcone, within the handle cavity or the clearance between the beam(s) and the interior of the hollow handle. Thus, by changing the various position of the beam within the handle, the stiffness of the racket and vibration damping can be modified.
In Dobo et al, the shaft depending from the head is held in an adjustable handle structure where the stiffness can be controlled by controlling the degree to which the shaft is gripped by an adjustable rubber, buffer device adjacent the top of the handle.
None of these attempts to effect racket structures suggest how to provide a consistent compromise between having a very rigid hitting platform and a more flexible racket structure so that while the shocks transmitted can be reduced, a pleasant and firm feel can be achieved for the player.